23 August 2015

David Clark Headphones amplifier design. 1

I was lucky enough to receive a pair of David Clark Headphones from my kids as a birthday gift. The model I have is the classic headset and mic designed for use in General Aviation cockpits and known as Type H10.13.4. This is the best selling headset in aviation. They fit tightly over the head and have a 33dB (@1000Hz) external attenuation.

For amateur radio use I found them to be a bit quiet and require me to turn up the AF Gain on my K2 Elecraft rig to near maximum volume before I could hear them.

I figured it would be a great project to try to build an audio booster amplifier from scratch.

The design in this blog post did not meet the objective.

The headset has an impedance of 150ohms. I determined through listening to a 1KHz tone that a peak-peak voltage of 1V maximum was required to drive the headset to a loud volume. However output distortion occurs when greater than 250mV. This distortion could be easily heard in the headphones. The reason has not been determined. Next I will try raising the emitter voltage to greater than 1V.

As a part of the design process a 150Ohm resistor was used. The results were the same when the headphones were actually connected to the output.

Design Input requirements:
  1. Rail voltage = between 7V and 12V to allow use of a 9V internal battery and an external 12V supply.
  2. Total current drain = <3mA.
  3. Frequency range = 200Hz - 3000Hz.
  4. Maximum input signal amplitude = 100mVp-p.
  5. Output signal amplitude = 1V.
  6. Load impedance = 150 Ohms.
The common emitter design is based on achieving a gain of 10. 

Calculated and installed component values are shown on the included schematic. 

Calculated and measured parameters are shown on the included schematic.

The 3dB filter roll-off points were measured as 200Hz and 1800Hz (approximately).

250mV Output for 22mV Input. Gain =11.4. No Distortion

800mV Output for 100mV Input. Voltage gain = 8. Distorted.

Prototype 1

20 August 2015

'Fun in the sun' with a true homebrew QRP field station. ZS6AZP

Thank you to my friend Dave ZS6AZP and the Centurion Radio Amateur Club ZS6CEN for allowing me to post a blog covering Dave's recent QRPexpedition. As usual Dave projects a 'no compromise' approach and a field operation in the true spirit of Amateur Radio (IMHO).

Check out the details at this link. Fun in the sun with ZS6AZP

I especially like the pool pole antenna support :)

Thanks Dave!

72 de Dick N4HAY

19 August 2015

Improving PCB enclosure building

I was recently inspired by the fantastic website of K7QO to try to do something to improve my PCB enclosure building, both from an efficiency perspective as well as from a neatness point of view.

I built a fixture in accordance with Chuck's directions on building enclosures and it turned out very well. Last night I was able to test solder together two sides using a piece of 24AWG insulated wire to ensure that the angle was 90 degrees once the solder dried (see pics below). I used a 30W Weller iron.  I found that the technique worked better by having the vertical piece of PCB mating directly with the bottom of the fixture and in front of the 24AWG wire.
Enclosure Fixture as per K7QO. The walls are a bit high. A lot of time was spent ensuring that the corner consisted of all 90 degree angles.

Much as I would love to own a shear I found that I could accurately cut PCB's to exact size using my table saw. More on that later when I get to build a prototype enclosure as the next step.

Thanks to Chuck for sharing his extensive knowledge with us.

A piece of insulated 24AWG wire is used to compensate for the solder shrinkage

Clamps are  used to hold the pieces in place

The 'shrinkage compensating angle' can be clearly seen

Completed prototype effort. In fact the thinner black wire resulted in a better right angle finish 

18 August 2015

CMoy pocket headphone amplifier measurements

Reference The CMoy headphone amplifier details

Gain and DC offset measurements were made using two different rail voltages as follows:

Input signal: A sinewave at 1kHz with variable amplitude. Source impedance = 50ohms.
Load: 270ohms on each channel to (simplistically) simulate the Beyerdynamic DT880 audiophile headphones owned by my son.
Both channels showed the same results.
LED current = 1.43mA
With rail-to-rail voltage = 7.68V, DC current = 9.1mA. (Same w/wo the 270ohm loads connected). Thus DC power = 70mW.
Vgnd-Vplus = 3.77V.  Vgnd-Vneg = -3.91V. Thus DC Offset = (7.68/2)-3.77=70mV.
At the point of clipping on positive voltage swings Vp-p(out) = 4.6V. Vp-p(in) = 430mV.
Thus Vgain = 10.69. Headroom on the Vplus rail = |(4.6/2)-3.77|=1.47V.
P=(Vp-p)^2/8R = 4.6^2/(8*270)=9.7mW.
Thus efficiency = 9.7/70 = 0.14.

With rail-to-rail voltage = 15.31V. DC current = 11mA. (Same w/wo the 270ohm loads connected). Thus DC power = 168mW.
Vgnd-Vplus = 7.59V.  Vgnd-Vneg = -7.72V. Thus DC Offset = (15.31/2)-7.59=65mV.
At the point of clipping on positive voltage swings Vp-p(out) = 10.9V. Vp-p(in) = 1V.
Thus Vgain = 10.9. Headroom on the Vplus rail = |(10.9/2)-7.59|=2.14V.
P=(Vp-p)^2/8R = 10.9^2/(8*270)=55mW.
Thus efficiency = 55/168 = 0.33

14 August 2015

CMoy pocket Headphone amplifier. Input coupling capacitor tests

My son recently became the proud owner of a pair of audiophile headphones. A pair of Beyerdynamic DT880 premiums. See the specs http://zs6rsh.blogspot.com/2015/07/headset-technical-specification.html

 Motivated by my friend Chris KD4PBJ, I built a prototype of the CMoy pocket headphone amplifier.  As specified, I used an OPA 2132PA Burr-Brown Op Amp. The rest of the components came out of my junkbox.

In reading the section on Input Capacitors for Headphone Amps, the author recommends the use of Polypropylene Film Capacitors as the best choice followed by Polyester Capacitors and to stay well away from Ceramic Capacitors. A very interesting read. I understand that a major issue in audiophile amps with AC coupling is Phase Distortion caused by the low frequency roll-off due to the RC high pass filter formed by the coupling capacitor and the amplifier input impedance. This causes a smearing of the Bass notes due to Phase delay from about 10Hz through 100Hz for a 0.1uF input coupling capacitor looking into an input impedance of 100Kohms.

Curious about this, I set up a test fixture to measure the phase distortion differences between what looked like a film poly capacitor in my junkbox (and which I deployed in my prototype) and a ceramic capacitor. I also ran an ideal model using LT spice. 

The high pass RC filter measured, consisted of the 0.1uF test capacitor and a 100Kohm resistor. As input I used a function generator set to approx 20mVp-p sinewave and two 10X probes connected to an oscilloscope to measure the phase delay between the input and output of the circuit. Lissajous figures were used to measure the phase angle to approximately a plus/minus 2 degree accuracy.

Attached find a graph of the plotted result. I noted that the measured -3dB filter knee for the poly capacitor was 17Hz plus/minus 2 Hz and the ceramic was 21Hz plus/minus 2Hz.

As can be seen, I was unable to measure a big difference in the phase delay of these capacitors using my 1970's era home lab equipment. The measured results compare favorably with the ideal capacitor phase delay calculated results.

In delving into the fascinating world of the audiophile at http://www.head-fi.org/ I came to realize that what constitutes a 'good sonic' is something highly complex and thus, highly subjective. So what is it that makes a polypropylene film capacitor 'sound' better than a ceramic capacitor? It is not revealed in my simplistic measurements. Perhaps driving the amp with a step signal (squarewave) would reveal much more about the differences in these capacitors?

The amp works very well with the DT880's to my ear although clipping of Bass notes was noticed when driven with 9volts. No clipping when using an 18volt source. Leading to an idea to build a buffer type virtual ground for the next version.

Thanks to the excellent website http://www.tangentsoft.net/, I have experienced a glimpse into the  complex world of the audiophile.

Now can I use one of these amplifiers to replace an LM386 in my regen receiver? :)

CMoy Prototype 1. The input and outputs should be isolated from the chassis!

The junkbox Wesco input capacitors can be seen. Next version needs 18volts to drive the DT880's. The Op Amp is a Burr-Brown OPA2132PA. Amazing sound! Zero hiss.

RC Network High Pass Filter test fixture. Input = 200mVp-p sinewave from a Function Generator (50ohm source impedance). Phase angles were measured using Lissajous figures. 10X scope probes used.

04 August 2015

Adventure Radio Society Spartan Sprint August 2015

Great fun was had in the Spartan Sprint Contest last night Monday evening.  Spartan Sprint Rules.

I operated my old trusty ATS3 rig into a BLTuner feeding an inverted Vee 80m doublet up about 55ft at the apex. Power output at the beginning of the contest was 2Watts on 40m and 20m. The battery was a 750mAH Li Po battery as the power with a set of mini paddles and earbuds. The total weight for this setup was 13.4oz (0.838lbs) of which 7oz was attributed to the ATU! There is definitely room for improvement and weight loss with my station.

QTH is near Raleigh NC operating from the kitchen table since it was extremely hot outside.

Band conditions were good and I was able to work all stations copied. 20m was excellent for the first hour and then switched to 40m. QRN was high on 40m but the signals were strong. Towards the last 30 minutes I switched back to 20m but no signals were copied. In particular I enjoyed the contacts with N4SX (1 Watt on 40m in GA) and KA5QQA (1 Watt on 20m in  TX).

Thanks to all for another great Spartan Sprint evening.


Total weight = 13.40oz = 0.838lbs
20m QSOs = 15
40m QSO's = 11
Total QSO's = 26

QSO's per Lb = 31.03